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Laser Remelting Produces Structured Surfaces

Photonics.comOct 2011
AACHEN, Germany, Oct. 4, 2011 — A method of structuring the metallic parts of tool inserts using laser remelting, rather than the more commonly used ablation, has been developed at the Fraunhofer Institute for Laser Technology ILT. This method gives tool manufacturers greater scope to adapt their production processes to incorporating novel structures and design elements more quickly and cost-effectively.

Surface with variable structures produced by laser remelting. (Images: Fraunhofer Institute for Laser Technology ILT)
Unlike laser structuring by ablation, remelting can directly produce finished surfaces that do not require any post-processing. For a structure depth of approximately 200 µm, this method can achieve processing rates of up to 75 mm in 2 min in a single pass. The ablation method, on the other hand, can achieve rates of only 1-10 mm in 3 min and requires roughly 10 passes to achieve a structure depth of 200 µm, which means that the laser-based structuring of large surfaces through ablation is generally not a cost-effective option for tool manufacturers.

The Fraunhofer ILT researchers said laser melting consumes fewer resources than the ablation method, and less energy is required for melting than for sublimation — the process requires significantly fewer passes — and there is no loss of material.
Dual-gloss effect: molded plastic component produced by a selectively laser-polished tool insert.
The laser melting method involves a laser beam traveling over the workpiece, and the resulting heat input melts the metal surface. At the same time, the laser power is modulated in order to continuously change the size of the melt pool at defined points.

“This modulation causes the material to be redistributed, creating mountains and valleys — half of the resulting structure lies above its initial level, while the other half lies below it,” said André Temmler, project manager at Fraunhofer ILT.

Thanks to surface tension, when the uppermost layer of the molten material solidifies, it exhibits uniformly low roughness, and the surface is left with a brilliant, polished finish. For flat surfaces and single-curved component geometries, the laser technique of structuring by remelting is already available for industrial use. Temmler and his team are now working on applying the technique to free-form surfaces.

In cases where an additional dual-gloss effect is required for end products such as decorative elements or an entire product surface, the first step is to apply a matte finish to the whole surface of the tool, which is generally achieved through blasting with glass beads or sand. Selected regions are then remelted using a laser beam. These regions solidify from the melt with a polished finish — in other words, the selective laser polishing creates a contrast between the matte, untreated areas and the brilliant, laser-polished areas. Depending on the intensity of the dual gloss, this can even produce a 3-D effect in which the polished points appear to protrude from the surface. For the first time, this selective polishing technique can be applied on an industrial scale for both flat and free-form surfaces.